1.4 Describe simple experiments leading to the idea of the small size of particles and their movement including dilution of coloured solutions and diffusion experiments 1.5 Understand the terms atom and molecule 1.6 Understand the differences between elements, compounds and mixtures 1.7 Describe techniques for the separation of mixtures, including simple distillation, fractional distillation, filtration, crystallization and paper chromatography
Dilution of Colored Solutions: Evidence of Particles
A serial dilution is a series of stepwise dilutions, for instance a blue copper sulphate solution, usually by the same factor e.g. ten times. the diluted solution is then diluted a thousand times or more by adding the same amount of water to it each time it is diluted.
A blue copper sulphate solution is made by dissolving crystals of copper sulphate in water. Serial dilution of such a blue copper sulphate solution produces solutions which are still blue.
Diffusion: Evidence of Particles
The following observations can only be explained if we accept the new idea that all matter is made up of particles which can move
A balloon leaks air as it goes flat after a few days
Making tea or coffee
Two gases mix evenly with each other
The smell food spreads from the kitchen
We can perfume/deodorant on a person
Solids dissolve in liquids e.g a sugar cube or a crystal of potassium permanganate; the solvent molecules cause the particles of the solid to separate and these dissolved solid particles then diffuse into the solvent
(this is the biology diffusion, but you get the point.)
Diffusion is the movement of particles with each particle moving in its own direction. This happens because particles with enough energy move from their positions i.e. particles in liquids and gases. Eventually particles from different substances will be mixed with each other.
For a substance to be able to diffuse its particles must be able to move freely. Although diffusion is fast in gases it still a slow process considering that gas particles move very fast.
Diffusion is slow because gas and liquid particles all move in different directions. Diffusion is a random process (it happens as a result of the random movement of the particles)
Diffusion is slower in all liquids than in gases because liquid particles are not as free to move as gas particles.
Gases and liquids can diffuse through solids but solids cannot diffuse through solids.
Atoms, Molecules, Elements, Compounds and Mixture
What are the small particles which make up substance? All substances are made up three different types of particles two of which are called atoms atoms and molecules. The smallest particle that can exist of a substance is called an atom. In many cases two or more atoms join together chemically to form a larger particle called a molecule.
We can divide up all substances into three groups by looking at the kind of particles in them. Elements
An element is a single substance that is made up of only one kind of atom and which cannot be split up into a simpler substance by any chemical process. More than 100 different elements have been identified, some of which have been made by man i.e. synthetic. Compounds
Compounds are substances which are made when two or more elements combined together chemically, they are the products of a chemical change. A compound is made up of molecules or ions.
A compound has different properties from the elements in it!
A compound also has more than two different types of particles/atoms/ions.
There are different types of compounds like organic and inorganic, acids, base and salts, oxides, hydroxides and so on. Mixtures
They are not chemically joint, but is joint physically. That is why they are easy to separate. Their features are slightly different from original elements.
Separating Mixtures
Often the substances that we want to use or study are part of mixtures. If we want to study their properties or use them, they need to be pure (i.e. one substance only) as usually impure change properties and make them less useful for our purpose.
You need to know the following separating techniques very well. This means that you should be able to describe how to carry out the separating technique (including being able to draw labelled diagrams of how to set up the apparatus) and know when to use it. For any given mixture you should be able to suggest the best separating technique.
Separating techniques use differences in properties between the substances that need separating. Some of these differences in properties are shown in the table below.
Separating Technique
How it works
Suitable solvent
Property used: Solubility
How? Select a solvent which dissolves some part of mixture but not the other part, separate by using filtration and then evaporate and let it crystallize.
Filtration
Property used: Different size of particles
How? Make mixture, then go through filter paper
Crystallization
Property used: Different boiling points
How? Heat, highest boiling point remains behind as crystals
Simple Distillation
Property used: Different boiling points
How? Boil mixture; lowest boiling point evaporates, catch steam and make it condense. (Condensation and evaporation)
The table below shows when each technique should be used
Separating Technique
Type of mixture
Example
Suitable Solvent
Two or more solids mixed together
Salt and Sand
Crystallization
Soluble solid and liquid - to obtain solid
Salt from saly water
Filtration
Insoluble solid and liquid
Sand and Water
Distillation
Soluble solid and liquid - to obtain liquid/solvent
Pure water from salty water
Fractional Distillation
Miscible Liquids
Ethanol and water, crude oil
Fractional DIstillation Two liquids
Two liquids (e.g. ethanol and water) with different boiling points may be separated by fractional distillation. The liquid with the lower boiling point (e.g. ethanol) evaporates more easily and rises through a fractionating column. Any gas/vapor of the liquid with the higher boiling point condenses back down. Only the vapor of the liquid with the lower boiling point reaches the top of the column and diffuses into the condenser. From here it is condensed and separated from the liquid with the higher boiling point. Mixture of Many Liquids E.g. Crude Oil
Crude oil is a mixture of hydrocarbons which boil at different temperatures. These hydrocarbons are different because they have different numbers of carbon atoms; this could be anything from 1 to well over 25. The lower the number of carbon atoms the lower their boiling point; the more volatile the hydrocarbons. A group of hydrocarbons with similar numbers of carbons and therefore similar boiling point is called a fraction. So each fraction has its own boiling point (or range of boiling points). The fraction with the lowest boiling points goes to the top of the fractionating tower where it is the coolest and are removed as gases. The other fractions also rise as vapor but liquids lower down the tower and are removed as volatile. The higher up the fractionating tower, the lower the number of fractions in the vapor mixture and the richer the vapor is in more volatile fractions.
Paper Chromatography
Dissolve sample usually by crushing it in the solvent using a pestle and mortar
Draw a line in pencil, called the base line on chromatography paper (should be pencil as carbon does not dissolve in any solvent)
Place a small drop of the sample on base line on chromatography paper.
Make a solution: If the sample is not a liquid then select a suitable solvent to make it into a solution: if insoluble in water, try ethanol or propanone.
Place chromatography paper in suitable solvent (solvent should not touch the sample on the paper).
Allow solvent to rise up the paper and dissolve the sample and to spread out the parts of the mixture on the chromatography paper.
Remove chromatography paper from beaker, allow the solvent to evaporate and interpret the chromatogram.
Interpreting a Chromatogram:
One spot on the chromatogram means that the sample was pure whilst more than one spot means it was a mixture
One spot remaining on the solvent line means the mixture/pure substance was not soluble in the liquid used and another solvent needs to be selected e.g. ethanol or propanone
The higher a sport, the more soluble the substance that made that spot
Locating Agent
When chromatography is used to analyze colorless substances, the chromatography is carried out as normal but at the end the chromatogram is treated with a locating agent before it is interpreted. The localing agent reacts with the colorless substances and changes them into colored substances.
HOME>SCIENCE>CHEMISTRY>1b: Atoms
1b: Atoms
1.4 Describe simple experiments leading to the idea of the small size of particles and their movement including dilution of coloured solutions and diffusion experiments1.5 Understand the terms atom and molecule
1.6 Understand the differences between elements, compounds and mixtures
1.7 Describe techniques for the separation of mixtures, including simple distillation, fractional distillation, filtration, crystallization and paper chromatography
Dilution of Colored Solutions: Evidence of Particles
A serial dilution is a series of stepwise dilutions, for instance a blue copper sulphate solution, usually by the same factor e.g. ten times. the diluted solution is then diluted a thousand times or more by adding the same amount of water to it each time it is diluted.
A blue copper sulphate solution is made by dissolving crystals of copper sulphate in water. Serial dilution of such a blue copper sulphate solution produces solutions which are still blue.
Diffusion: Evidence of Particles
The following observations can only be explained if we accept the new idea that all matter is made up of particles which can move
Diffusion is the movement of particles with each particle moving in its own direction. This happens because particles with enough energy move from their positions i.e. particles in liquids and gases. Eventually particles from different substances will be mixed with each other.
For a substance to be able to diffuse its particles must be able to move freely. Although diffusion is fast in gases it still a slow process considering that gas particles move very fast.
Diffusion is slow because gas and liquid particles all move in different directions. Diffusion is a random process (it happens as a result of the random movement of the particles)
Diffusion is slower in all liquids than in gases because liquid particles are not as free to move as gas particles.
Gases and liquids can diffuse through solids but solids cannot diffuse through solids.
Atoms, Molecules, Elements, Compounds and Mixture
What are the small particles which make up substance? All substances are made up three different types of particles two of which are called atoms atoms and molecules. The smallest particle that can exist of a substance is called an atom. In many cases two or more atoms join together chemically to form a larger particle called a molecule.
We can divide up all substances into three groups by looking at the kind of particles in them.
Elements
An element is a single substance that is made up of only one kind of atom and which cannot be split up into a simpler substance by any chemical process. More than 100 different elements have been identified, some of which have been made by man i.e. synthetic.
Compounds
Compounds are substances which are made when two or more elements combined together chemically, they are the products of a chemical change. A compound is made up of molecules or ions.
A compound has different properties from the elements in it!
A compound also has more than two different types of particles/atoms/ions.
There are different types of compounds like organic and inorganic, acids, base and salts, oxides, hydroxides and so on.
Mixtures
They are not chemically joint, but is joint physically. That is why they are easy to separate. Their features are slightly different from original elements.
Separating Mixtures
Often the substances that we want to use or study are part of mixtures. If we want to study their properties or use them, they need to be pure (i.e. one substance only) as usually impure change properties and make them less useful for our purpose.
You need to know the following separating techniques very well. This means that you should be able to describe how to carry out the separating technique (including being able to draw labelled diagrams of how to set up the apparatus) and know when to use it. For any given mixture you should be able to suggest the best separating technique.
Separating techniques use differences in properties between the substances that need separating. Some of these differences in properties are shown in the table below.
Separating Technique
How it works
Fractional DIstillation
Two liquids
Two liquids (e.g. ethanol and water) with different boiling points may be separated by fractional distillation. The liquid with the lower boiling point (e.g. ethanol) evaporates more easily and rises through a fractionating column. Any gas/vapor of the liquid with the higher boiling point condenses back down. Only the vapor of the liquid with the lower boiling point reaches the top of the column and diffuses into the condenser. From here it is condensed and separated from the liquid with the higher boiling point.
Mixture of Many Liquids E.g. Crude Oil
Crude oil is a mixture of hydrocarbons which boil at different temperatures. These hydrocarbons are different because they have different numbers of carbon atoms; this could be anything from 1 to well over 25. The lower the number of carbon atoms the lower their boiling point; the more volatile the hydrocarbons. A group of hydrocarbons with similar numbers of carbons and therefore similar boiling point is called a fraction. So each fraction has its own boiling point (or range of boiling points). The fraction with the lowest boiling points goes to the top of the fractionating tower where it is the coolest and are removed as gases. The other fractions also rise as vapor but liquids lower down the tower and are removed as volatile. The higher up the fractionating tower, the lower the number of fractions in the vapor mixture and the richer the vapor is in more volatile fractions.
Paper Chromatography
- One spot on the chromatogram means that the sample was pure whilst more than one spot means it was a mixture
- One spot remaining on the solvent line means the mixture/pure substance was not soluble in the liquid used and another solvent needs to be selected e.g. ethanol or propanone
- The higher a sport, the more soluble the substance that made that spot
Locating AgentWhen chromatography is used to analyze colorless substances, the chromatography is carried out as normal but at the end the chromatogram is treated with a locating agent before it is interpreted. The localing agent reacts with the colorless substances and changes them into colored substances.